Yes, bad coverage may cause a lot more of battery usage. But generally 2G coverage is better than 3G coverage at any point. This is because 2G works fine with stations much further away. So, as such setting phone to GSM mode only will most often help in case of bad coverage.

Here in Finland, there is 2G coverage generally anywhere, even in the forests etc. But 3G coverage is only in the places where there are lots of people nearby. That is both because 2G was there before 3G but also due to it being cheaper to fill given area with 2G coverage than with 3G coverage.